The present invention generally relates to a voltage controlled oscillator (VCO), and, more particularly, to a VCO in Colpitts and/or Clapp configuration with varactors.
Voltage controlled oscillators (VCO) are designed to produce an oscillating signal (xe2x80x9ccarrier signalxe2x80x9d) of a particular frequency F. Typically, VCOs are used in radio receiver and transmitter circuits. VCOs often comprise tank circuits with inductors as well as with variable capacitors. These capacitors are usually implemented by varactor diodes (in short xe2x80x9cvaractorsxe2x80x9d or xe2x80x9cvaricapxe2x80x9d).
The frequency F of the oscillating signal is determined by the magnitude of a tuning voltage VTUNE applied to the varactor (to its pn-junction). The frequency F can be varied reliably between a certain maximum frequency FMAX and a certain minimum frequency FMIN, often a radio frequency (RF). The frequencies between these limits are referred to as the VCO""s frequency range (or xe2x80x9cbandwidthxe2x80x9d). The VCO tuning sensitivity is defined as frequency change over tuning voltage change (i.e. derivation dF/dVTUNE). It is desired to tune the oscillator over a large frequency range with a small tuning voltage range.
However, there are also particular requirements for low noise performance over the complete frequency range. Noise that appears around the oscillator frequency can lead to unwanted phase or frequency modulation of the oscillating signal; this so-called phase noise is a relative power usually measured in a 1 Hz bandwidth, at a measurement frequency that is offset (e.g., 25 kHz) to the frequency F of the oscillating signal (single side-band phase noise), the unit is xe2x80x9cdBc in 1 Hzxe2x80x9d.
Resistive losses especially those in the inductors and varactors are of major importance and determine the quality factor Q of each tank circuit; generally, a high Q is desirable. With the desire to provide small VCOs (e.g., in portable radios), the achievable Q decreases, thus leading to further unwanted effects such as higher noise and a small frequency range.
Since each VCO needs an amplifying element combined with a feedback network, the open loop gain of that arrangement might vary (i.e. increase) with the frequency. Also, quality factor Q variations might cause an unwanted signal level change of the tank circuit that further influences the performance of the VCO.
Both such gain and Q variations might lead to an unwanted increase of the oscillating signal amplitude at higher frequencies; this needs to be accommodated, for example, by regulating means.
Also, with increasing frequency range, the noise might be more frequency dependent, the power consumption might rise, and the output power might fall. In other words, the overall performance is optimal for few frequencies only.
The following references are useful: U.S. Pat. No. 1,624,537 (Colpitts); U.S. Pat. No. 2,756,334 (Blum); U.S. Pat. No. 2,225,897 (Bell); U.S. Pat. No. 4,621,241 (Kiser); U.S. Pat. No. 4,785,263 (Kaltenecker et al.); U.S. Pat. No. 5,144,264 (Chong et al.), U.S. Pat. No. 5,959,504 (Wang); U.S. Pat. No. 5,982,243 (Pope), U.S. Pat. No. 6,046,647 (Nelson); U.S. Pat. No. 6,081,167 (Kromat); as well as WO99/53608 (Lichterfeld).
There is a need to find a simple VCO that allows a good compromise between performance and limited space.